The present invention relates to optical detection and, more particularly, but not exclusively, to method and device for detecting weak optical signals, e.g., for the purpose of imaging.
Recording and measuring a weak signal presents challenging and acute problems for the designers of modern sensors for myriad applications in diverse fields of science and technology. In these sensors, various primary signals (optical, ultrasonic, mechanical, chemical, radiation, etc.) are transformed into elementary charge carriers, such as electrons, holes or ions. Signal charge packets of such elementary charge carriers are amplified and converted to an electrical signal which is fed into a recording or analyzing device and/or used as a feedback signal for monitoring.
In many applications, such as those relating to imaging systems, sensor devices with critical threshold parameters are in an acute demand. Such applications demand sensors capable of detecting and recording of optical signals that are not only weak, but also short in duration and/or rapidly varying. Accordingly, these applications require a sensor capable of amplifying such electrical signals over a wide bandwidth and with a low noise level. Optical signal amplification are evaluated according to variety of parameters, such as, signal resolution, threshold sensitivity, response speed, complexity, physical size, physics principle, power consumption, manufacturing technology, reliability, cost and the like. Many of these parameters are not independent and high-performance systems are usually more complex and expensive.
One approach to the detection of weak optical signals is the use of photodetectors in which the exposure times are long. These photodetectors typically employ semiconductor technology. Long exposure time photodetectors are suitable for static light source having constant intensity over time (e.g., stars), but are not suitable for rapid imaging applications in which the light has non constant emission intensity and/or originate from moving objects.
A known problem with long exposure time photodetectors is that thermally induced currents exist in the semiconductor even in the absence of incident optical signal. Such current is referred to in the literature as “dark current.” Attempts have been made to devise low dark current detectors, which are typically cauterized by high QE and high Fill Factor (FF) so as to minimize the signal loss. Exposure periods up to approximately one hour are achievable by cooling the detector typically by means of thermoelectric coolers. Such detectors, however, are very expensive and usually used in the area of astronomy research.
Another approach employs avalanche amplification (multiplication) of charge carriers. To date, avalanche amplification is recognized as a highly sensitive and high-speed method of amplification. Avalanche amplification is based on impact ionization arising in a strong electric field. The charge carriers accelerate in the electric field and ionize the atoms of the working medium of the amplifier, resulting in multiplication of the charge carriers. At a high multiplication factor, however, it is difficult to stabilize the avalanche amplification operating point. Additionally, the internal noise level and the response time grow rapidly with the multiplication factor.
Avalanche amplification based photodetectors are capable of converting a single photon to charge carriers and multiplying the charge. The number of photonic events is statistically estimated using the known QE of the device. These photodetectors are suitable for static and well as dynamic light sources. Representative examples of such photodetectors include, high resolution arrays of photomultiplier tubes, avalanche photodiode array activated in the Geiger mode, electron multiplied CCDs, and intensified image sensors.
Photomultiplier tubes are bulky devices constructed from a vacuum tube which houses a photocathode, an electron multiplier and an anode. Incident photons strike the photocathode material thereby producing electrons under the photoelectric effect. These electrons are directed towards the electron multiplier, where electrons are multiplied by the process of secondary emission. The electrons are accumulated at the anode resulting in a sharp current pulse indicating the arrival of a photon. For the very low resolution imaging, multi-anode photomultiplier tubes are available; however, they are extremely costly and their resolution is far from imaging demands.
Avalanche photodiodes are the semiconductor analog to the photomultiplier tubes. By applying a high reverse bias voltage, an avalanche photodiode presents an internal current gain effect due to impact ionization. Unlike the photomultiplier tube, an array of the avalanche photodiode provides high resolution imaging with medium cost effectiveness. However, these devices suffer from high dark current and therefore require cooling to cryogenic temperatures for single photon imaging. The cooling requirement presents a major drawback to the technology because the cooling system significantly increases the power consumption, dimensions and cost of the device.
Electron multiplying CCDs combine high photon conversion efficiency (up to 90%) with reduction of the readout noise. The technology is used in high-end applications such as low-light dynamic cellular microscopy and single molecule detection. The electron multiplying CCD does not require high voltage supply; however, similarly to the avalanche photodiodes, the single photon detection can only be achieved if the device is cooled to cryogenic temperature.
Image intensified sensors are based on more promising technology. Most sensors employ a CCD and an image intensifier that is fiber optically coupled to the CCD to increase the sensitivity down to single photon level. Other sensors employ a CMOS image sensor instead of a CCD. Unlike the CCD, the CMOS image sensor already includes circuitry therein. The image intensified sensors are expensive, relatively bulky and power consuming. Moreover, multiple optical interfaces in the coupling between the image intensifier and the CCD or CMOS results in image degradation.
Another type of image intensified sensors addresses the problem of image degradation by employing a process known as electron-bombarded semiconductor gain. One such image intensified sensor is the Electron Bombarded CCD (EBCCD) which consists of a back illuminated CCD used as an anode in proximity focus with the photocathode. The term “proximity focus” is known in the art and referrers to a configuration in which the photocathode and the anode are spaced closely together. Photoelectrons from the photocathode are accelerated to and directly imaged in the back illuminated CCD which is enclosed into vacuum enclosure. The gain is achieved by the electron-bombarded impact ionization inducing signal related charge multiplication (low noise semiconductor gain process). The EBCCD eliminates the necessity in micro channel plate phosphor screen, and fiber optics tapers of the image intensifiers. An EBCCD is disclosed in U.S. Pat. No. 4,687,922.
Conventional EBCCD suffer from several limitations. When the CCD is of frame-transfer type, the vacuum enclosure volume is relatively large. On the other hand, when the CCD is of interline-transfer, the vacuum enclosure volume is smaller but a mechanical shutter is required.
An additional limitation is the complicated manufacturing process which requires specialized processing to provide thin semiconductor and to passivate the back surface. A further limitation is the relatively large dimension of the device.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and device for detecting weak optical signals devoid of the above limitations.